The present invention relates to wire grid polarizers in general and in particular to multilayer wire grid polarizers and beamsplitters for the visible spectrum.
The use of an array of parallel conducting wires to polarize radio waves dates back more than 110 years. Wire grids, generally in the form of an array of thin parallel conductors supported by a transparent substrate, have also been used as polarizers for the infrared portion of the electromagnetic spectrum.
The key factor that determines the performance of a wire grid polarizer is the relationship between the center-to-center spacing, sometimes referred to as period or pitch, of the parallel grid elements and the wavelength of the incident light. If the grid spacing or period is long compared to the wavelength, the grid functions as a diffraction grating, rather than as a polarizer, and diffracts both polarizations, not necessarily with equal efficiency, according to well-known principles. However, when the grid spacing (p) is much shorter than the wavelength, the grid functions as a polarizer that reflects electromagnetic radiation polarized parallel (xe2x80x9csxe2x80x9d polarization) to the grid, and transmits radiation of the orthogonal polarization (xe2x80x9cpxe2x80x9d polarization).
The transition region, where the grid period is in the range of roughly one-half of the wavelength to twice the wavelength, is characterized by abrupt changes in the transmission and reflection characteristics of the grid. In particular, an abrupt increase in reflectivity, and corresponding decrease in transmission, for light polarized orthogonal to the grid elements will occur at one or more specific wavelengths at any given angle of incidence. These effects were first reported by Wood in 1902, and are often referred to as xe2x80x9cWood""s Anomalies.xe2x80x9d Subsequently, in 1907, Rayleigh analyzed Wood""s data and had the insight that the anomalies occur at combinations of wavelength and angle where a higher diffraction order emerges. Rayleigh developed the following equation to predict the location of the anomalies, which are also commonly referred to in the literature as xe2x80x9cRayleigh Resonancesxe2x80x9d.
xcex=xcex5(n+/xe2x88x92sin xcex8)/kxe2x80x83xe2x80x83(1)
where epsilon (xcex5) is the grating period; n is the refractive index of the medium surrounding the grating; k is an integer corresponding to the order of the diffracted term that is emerging; and lambda and theta are the wavelength and incidence angle (both measured in air) where the resonance occurs.
For gratings formed on one side of a dielectric substrate, n in the above equation may be equal to either 1, or to the refractive index of the substrate material. Note that the longest wavelength at which a resonance occurs is given by the following formula:
xcex=xcex5(n+sin xcex8)xe2x80x83xe2x80x83(2)
where n is set to be the refractive index of the substrate.
The effect of the angular dependence is to shift the transmission region to larger wavelengths as the angle increases. This is important when the polarizer is intended for use as a polarizing beam splitter or polarizing turning mirror.
In general, a wire grid polarizer will reflect light with its electric field vector parallel (xe2x80x9csxe2x80x9d polarization) to the wires of the grid, and transmit light with its electric field vector perpendicular (xe2x80x9cpxe2x80x9d polarization) to the wires of the grid, but the plane of incidence may or may not be perpendicular to the wires of the grid as discussed here. Ideally, the wire grid polarizer will function as a perfect mirror for one polarization of light, such as the S polarized light, and will be perfectly transparent for the other polarization, such as the P polarized light. In practice, however, even the most reflective metals used as mirrors absorb some fraction of the incident light and reflect only 90% to 95%, and plain glass does not transmit 100% of the incident light due to surface reflections. The performance of wire grid polarizers, and indeed other polarization devices, is mostly characterized by the contrast ratio, or extinction ratio, as measured over the range of wavelengths and incidence angles of interest. For a wire grid polarizer or polarization beamsplitter, the contrast ratios for the transmitted beam (Tp/Ts) and the reflected beam (Rs/Rp) may both be of interest.
Historically, wire grid polarizers were developed for use in the infrared, but were unavailable for visible wavelengths. Primarily, this is because processing technologies were incapable of producing small enough sub-wavelength structures for effective operation in the visible spectrum. Nominally, the grid spacing or pitch (p) should be less than xcx9cxcex/5 for effective operation (for pxcx9c0.10-0.13 xcexcm for visible wavelengths), while even finer pitch structures (pxcx9cxcex/10 for example) can provide further improvements to device contrast. However, with recent advances in processing technologies, including 0.13 xcexcm extreme UV photolithography and interference lithography, visible wavelength wire grid structures have become feasible. Although there are several examples of visible wavelength wire grid polarizers devices known in the art, these devices do not provide the very high extinction ratios ( greater than 1,000:1) across broadband visible spectra needed for demanding applications, such as for digital cinema projection.
An interesting wire grid polarizer is described by Garvin et al. in U.S. Pat. No. 4,289,381, in which two or more wire grids residing on a single substrate are separated by a dielectric interlayer. Each of the wire grids are deposited separately, and the wires are thick enough (100-1000 nm) to be opaque to incident light. The wire grids effectively multiply, such that while any single wire grid may only provide 500:1 polarization contrast, in combination a pair of grids may provide 250,000:1. This device is described relative to usage in the infrared spectrum (2-100 xcexcm), although presumably the concepts are extendable to visible wavelengths. However, as this device employs two or more wire grids in series, the additional contrast ratio is exchanged for reduced transmission efficiency and angular acceptance. Furthermore, the device is not designed for high quality extinction for the reflected beam, which places some limits on its value as a polarization beam splitter.
A wire grid polarization beam splitter for the visible wavelength range is described by Hegg et al. in U.S. Pat. No. 5,383,053, in which the metal wires (with pitch p less than  less than xcex and xcx9c150 nm features) are deposited on top of metal grid lines, each of which are deposited onto a glass or plastic substrate. Wile this device is designed to cover much of the visible spectrum (0.45-0.65 xcexcm), the anticipated polarization performance is rather modest, delivering an overall contrast ratio of only 6.3:1.
Tamada et al, in U.S. Pat. No. 5,748,368, describes a wire grid polarizer for the near infrared spectrum (0.8-0.95 xcexcm) in which the structure of the wires is shaped in order to enhance performance. In this case, operation in the near infrared spectrum is achieved with a wire structure with a long grid spacing (xcex/2 less than p less than xcex) rather than the nominal small grid spacing (pxcx9cxcex/5) by exploiting one of the resonances in the transition region between the wire grid polarizer and the diffraction grating. The wires, each xcx9c140 nm thick, are deposited on a glass substrate in an assembly with wedge plates. In particular, the device uses a combination of trapezoidal wire shaping, index matching between the substrate and a wedge plate, and incidence angle adjustment to tune the device operation to hit a resonance band. While this device provides reasonable extinction of xcx9c35:1, which would be useful for many applications, this contrast is inadequate for applications, such as digital cinema, which require higher performance. Furthermore, this device only operates properly within narrow wavelength bands (xcx9c25 nm) and the device is rather angularly sensitive (a 2xc2x0 shift in incidence angle shifts the resonance band by xcx9c30 nm). These considerations also make the device unsuitable for broadband wavelength applications in which the wire grid device must operate in xe2x80x9cfastxe2x80x9d optical system (such as F/2).
Most recently, U.S. Pat. No. 6,108,131 (Hansen et al.) and U.S. Pat. No. 6,122,103 (Perkins et al.), both assigned to Moxtek Inc. of Orem, Utah, describe wire grid polarizer devices designed for the visible spectrum. Accordingly, the ""131 patent describes a straightforward wire grid polarizer designed to operate in the visible region of the spectrum. The wire grid nominally consists of a series of individual wires fabricated directly on a substrate with a xcx9c0.13 xcexcm gridline spacing (pxcx9cxcex/5), wire nominal width of 0.052-0.078 xcexcm wide (w), and wire thickness (t) greater than 0.02 xcexcm. By using wires of xcx9c0.13 xcexcm grid spacing or pitch, this device has the required sub-visible wavelength structure to allow it to generally operate above the long wavelength resonance band and in the wire grid region. The ""103 patent proposes a variety of improvements to the basic wire grid structure directed to broadening the wavelength spectrum and improving the efficiency and contrast across the wavelength spectrum of use without requiring finer pitch structures (such as xcx9cxcex/10). Basically, a variety of techniques are employed to reduce the effective refractive index (n) in the medium surrounding the wire grid, in order to shift the longest wavelength resonance band to shorter wavelengths (see equations (1) and (2)). This is accomplished most simply by coating the glass substrate with a dielectric layer which functions as an anti-reflectional (AR) coating, and then fabricating the wire grid onto this intermediate dielectric layer. The intermediate dielectric layer effectively reduces the refractive index experienced by the light at the wire grid, thereby shifting the longest wavelength resonance shorter. The ""103 patent also describes alternate designs where the effective index is reduced by forming grooves in the spaces between the wires, such that the grooves extend into the substrate itself, and/or into the intermediate dielectric layer which is deposited on the substrate. As a result of these design improvements, the low wavelength band edge shifts xcx9c50-75 nm lower, allowing coverage of the entire visible spectrum. Furthermore, the average efficiency is improved by xcx9c5% across the visible spectrum over the basic prior art wire grid polarizer.
While the devices described in U.S. Pat. Nos. 6,108,131 and 6,122,103 are definite improvements over the prior art, there are yet further opportunities for performance improvements for both wire grid polarizers and polarization beam splitters. In particular, for optical systems with unpolarized light sources, where system light efficiency must be maximized, polarization beam splitters which provide high extinction of both the reflected and transmitted beams are valuable. As the commercially available wire grid polarizers from Moxtek provide only xcx9c20:1 contrast for the reflected channel, rather than 100:1 or even 2,000:1, its utility is limited. Additionally, the performance of these devices varies considerably across the visible spectrum, with the polarization beam splitter providing contrast ratios for the transmitted beam varying from xcx9c300:1 to xcx9c1200:1 from blue to red, while the reflected beam contrast ratios vary from 10:1 to 30:1. Thus there are opportunities to provide polarization contrast performance in the blue portion of the visible spectrum in particular, as well as more uniform extinction across the visible. Finally, there are also opportunities to improve the polarization contrast for the transmitted p-polarization light beyond the levels provided by prior art wire grid devices. Such improvements would be of particular benefit for the design of electronic imaging systems, such as electronic projection systems, including those for digital cinema.
Thus, there exists a need for an improved wire grid polarizer, particularly for use in visible light systems requiring broad wavelength bandwidth and high contrast (target of 1,000:1 or greater). In addition, there exists a need for such an improved wire grid polarizer for use at incidence angles of about 45 degrees.
Briefly, according to one aspect of the present invention a wire grid polarizer for polarizing an incident light beam comprises a substrate having a first surface. A grid or array of parallel, elongated, conductive wires is disposed on the first surface, and each of the adjacent wires are spaced apart on a grid period less than a wavelength of incident light. Each of the wires comprises an intra-wire substructure of alternating elongated metal wires and elongated dielectric layers.